Theophylline inhibits cyclic nucleotide phosphodiesterase and potentiates cyclic nucleotide dependent processes in many tissues. This methylxanthine greatly prolongs the afterhyperpolarization of a directly evoked action potential in bullfrog sympathetic ganglion cells. Cyclic nucleotides could therefore be involved in this process. This possibility was investigated by means of the sucrose gap technique. Theophylline increased the concentration of cyclic AMP and cyclic GMP in these ganglia. However a variety of other agents which elevated cyclic nucleotides in these ganglia did not augment the spike after-hyperpolarization. Application of high doses of cyclic nucleotides did not affect the spike afterhyperpolarization. Additionally, following inhibition of phosphodiesterase by 10 to the minus 4th power M papaverine, theophylline still potentiated the spike afterhyperpolarization. It therefore appears that the mechanism whereby theophylline potentiates the spike afterhyperpolarization is independent of the cyclic nucleotides.